This invention relates to dynamoelectric machines, and in particular, to single phase multi-speed induction motors. While the invention is described with particular reference to motors used in hermetic compressor applications, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
There has long been a need in the residential refrigeration market for efficient and economical equipment. The rising cost of energy, in all forms, has tended to accent this need. One area where improved efficiency can be obtained is in the sizing of the equipment itself so that efficient operation is attained over a wide range of load conditions. In the past, it has been difficult to size the equipment so that the equipment, as installed, has ample capacity for peak load conditions, and also operates efficiently and provides comfort at lighter load conditions. Prior art solutions to the sizing problems usually involve various mechancial unloading devices in order to regulate the system. The terms refrigeration and air conditioning are herein used interchangeably in their broadest generic sense, and are intended to include any system having an electric induction motor as an element.
Recently, attempts have been made to modulate the compressor of the refrigeration system by operating the compressor at two distinct speeds. One example of a two speed motor useful in this modulation is shown and described in U.S. Pat. No. 4,103,212, granted July 25, 1978 and assigned to the assignee of the present invention. Briefly, this two speed motor described in the above-noted U.S. patent is a so-called consequent pole motor and its low speed torque is fixed at about half (i.e., about 55 percent) of its high speed torque.
In general, multiple speed motors are well-known in the art. In the past, multiple speed motors conventionally have been of two types, viz, distinct winding motors and consequent pole motors. So-called distinct winding single phase motors are constructed by placing a plurality of distinct windings within a stator core, and thereafter switching between the sets of distinct windings to vary the motor speed. For the purposes of this specification, the term "distinct winding" means that each main winding of the motor has a corresponding auxiliary winding used only in conjunction with its main winding. With this type of motor construction, the number of main winding poles conventionally equals the number of auxiliary winding poles and there is a winding set consisting of one main and one auxiliary winding for each operating speed of the motor.
While these distinct winding, multiple speed motors work well for their intended purposes, and while the torque of the motor in its low speed mode of operation can be selected to be any desired amount of its high speed torque by varying the turns in the distinct windings, distinct winding motors generally have been used in applications where the slot fill of the motor is not critical. "Slot fill" is a term of art, and generally refers to the slot area displaced by the turns of the motor windings divided by the total usable slot area, expressed as a percentage. In many induction motor applications, slot fills are not critical, and ample slot space is provided in the lamination design for carrying a number of motor windings in the slots of the stator assembly. However, since separate main and auxiliary windings must be provided for each motor speed, distinct winding motors have presented problems in motor applications where slot fill is critical. Also, distinct winding motors require a considerable amount of magnet wire for the distinct windings which adds to the cost of the motor.
Hermetic motors, on the other hand, usually require slot fill concentrations so as to result in the highest possible output for the smallest motor volume. This generally precludes the use of multiple independent or distinct windings. In general, high slot fills are desirable to achieve efficient motor operation, and motor performance can be improved by increasing the amount of material used in the windings of the stator. This expedient is practiced extensively in hermetic motor design. However, this adds to the manufacturing cost of the motor.
An additional factor involved in motor design for hermetic motors is the fact that the hermetic motor is enclosed and hermetically sealed within the compressor unit of the refrigeration system. Electrical connections are made through the shell of the compressor. The shell has a connection opening therethrough, and a special connector that preserves the integrity of the refrigerant system is inserted in and hermetically seals the opening. The use and insertion of the connectors in the shell add significantly to the cost of the compressor. Consequently, a general design requirement is that motors utilized in hermetic compressors should incorporate a minimum number of leads so as to minimize construction problems and the cost inherent in making multiple electrical connector openings through the compressor shell.
In multiple speed motors, the motor usually has its highest torque (and hence highest horsepower output) when operated at its high speed mode of operation. For example, in a two pole/four pole motor, the motor may be operated at approximately 3,600 rpm (ignoring slip) when in its high speed mode and approximately 1,800 rpm when in its low speed mode. However, as heretofore mentioned, the torque output of a conventional consequent pole motor, when operated in its low speed mode is only about half (e.g., 55 percent) of its torque output when operated in its high speed mode. Since the power output of a motor is directly proportional to the product of the motor speed and the torque of the motor, the power output of such consequent pole motors when operated at low speed is only about 1/4 of its power when operated at high speed. In many motor applications and under many load conditions, such consequent pole multiple speed motors do not develop sufficient torque (or horsepower) when operated at low speed.
As mentioned above, while the torque of distinct winding motors may be of any desired output when the motor is operated in low speed, these distinct winding motors have high slot fills and require such additional material that they cannot be effectively utilized in many hermetic motor applications. Hence, there has been a need for a two speed motor which develops more torque than a conventional consequent pole two speed motor when operated in its low speed mode of operation, and yet which is less expensive to manufacture, which is more compact, and which has lower slot fills than prior art distinct winding multiple speed motors.
For additional background information, reference may be made to my co-assigned U.S. Pat. No. 4,103,213 issued July 25, 1978, which discloses a two-speed, single phase consequent pole induction motor.
Among the several objects and features of the present invention may be noted the provision of a multiple speed, single phase induction motor or other dynamoelectric machine which can be readily designed to give any desired breakdown torque (i.e., maximum torque) during its low speed operational mode which is less than, equal to, or greater than a desired breakdown torque of the motor when operated in its high speed mode;
The provision of such a motor which efficiently uses the material incorporated in the motor and is therefore less expensive to manufacture;
The provision of such a motor in which the slot fills of the stator core are not excessive;
The provision of such a motor which does not require separate or distinct main and auxiliary windings for each operational speed of the motor;
The provision of such a motor which has a minimum number of leads required for energization of the motor;
The provision of a method of constructing a two speed electric motor which, during design of the motor, enables the breakdown torque of the motor, when operated in its low speed mode, to be of virtually any desired value less than, equal to, or greater than a desired high speed breakdown torque of the motor;
The provision of such a method which enables the production of such two speed motors with conventional production equipment;
The provision of such a method which may be used to manufacture any multi-speed motor having twice as many electrical poles when operated at low speed as when operated at high speed;
The provision of such a motor which enables the breakdown torque of the motor to be matched to the load on the motor when the motor is operated in both, its low and high speed modes; and
The provision of such a motor which is economical to manufacture, which is efficient in operation, and which has a long service life.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.